This invention relates to an optical receiver used for a digital optical transmission system.
A conventional optical receiver of the type described is classified into a first optical receiver for receiving a burst signal and a second optical receiver for receiving a continuous signal. The first optical receiver for the burst signal comprises a photo diode and a plurality of limit amplifier circuits connected In series to one another. Each limit amplifier circuit has an offset compensating function determined by an offset compensation time constant. Disclosure is made about such an optical receiver comprising the plurality of limit amplifier circuits, for example, in Japanese Unexamined Patent Publication (JP-A) No.310967/1994, Japanese Unexamined Patent Publication (JP-A) No. 84160/1996, or Proceedings of the 1996 IEICE (THE INSTITUTE OF ELECTRONICS, INFORMATION AND COMMUNICATION ENGINEERS) General Conference C-588. Because the optical receiver capable in response to the burst signal is required to carry out offset compensation at a high speed, the offset compensation time constant of each of the limit amplifier circuits is set to a small value.
On the other hand, the second conventional optical receiver for receiving only a continuous signal has a preamplifier which is connected to a post-amplifier through a coupling capacitor for unipolar-to-bipolar transformation. The post amplifier comprises a plurality of limit amplifier circuits connected in series to one another. Furthermore, the post amplifier compensates internal offset of its circuit by having a negative feedback circuit which has an integrating circuit and which maintains a time average value of positive and negative outputs at a constant value.